The present invention relates to a control of fuel to be supplied to an internal-combustion engine.
Conventionally, in order to regenerate an exhaust gas purifying device provided in an exhaust system of an engine, an air/fuel ratio is switched from lean to rich at a predetermined timing. Japanese Patent Application Publication No. 2003-322015 discloses a technique for performing a post-injection of a small amount of fuel after a main injection so as to reduce NOx trapped in a NOx trapping catalyst disposed in an exhaust system of an engine. According to this technique, a fuel amount to be injected in the post-injection and its injection timing are determined based on a delay of an actual value with respect to a desired value for the intake air amount.
The post-injection is capable to enrich the air/fuel ratio to reduce NOx without any torque variation. However, if such a post-injection is performed under a state of air shortage, the amount of unburned fuel to be exhausted, that is, the exhaust amount of HC (hydrocarbon) may increase because the post-injection is performed in an expansion stroke or an exhaust stroke.
Considering these problems, the present invention aims at suppressing an increase of the HC exhaust amount even when an injection for reducing an exhaust gas purifying device is performed under a state of air shortage.